1. Field of the Invention
The present invention relates to a fabrication method of an optical semiconductor device and, particularly, to a fabrication method of a semiconductor laser and an optical modulator for use in a WDM (wavelength division multiplexing) transmission system.
2. Description of the Related Art
Research and development of optical communication systems using WDM for transmitting optical signals having different wavelengths through a single optical fiber have been actively made as a means for substantially expanding the optical communication capacity using the existing optical fiber network. In the basic trunk network into which the WDM system is mainly introduced, optical signals modulated at a speed not lower than several G bits/sec are transmitted through the optical fiber at least several hundreds kilometers long. Therefore, a DFB (distributed feedback) laser having high spectral purity has been utilized as a light source and an external modulation system in which wavelength is not varied during a modulating operation has been used in modulating the optical signal. As a modulator for use in the external modulation, a semiconductor EA (electro-absorption) modulator or a LN (lithium niobate) modulator has been utilized. Particularly, since the EA modulator can be monolithically integrated with the DFB laser as the light source, it is effective in reducing the size and power consumption and is the key device of a high density WDM system. Since the basic purpose of the WDM system is to transmit optical signals having different wavelengths through a single optical fiber, DFB lasers having various oscillation wavelengths are required as light sources and optical modulators corresponding to the respective wavelengths are required. In a fabrication method of a usual EA modulator integrated DFB laser, an element having only one oscillation wavelength or operation wavelength can be obtained from one semiconductor substrate. In view of this fact, a method for forming optical semiconductor devices having different operation wavelengths (operation energies) on a semiconductor substrate as an optical semiconductor element adaptable to the WDM has been proposed in K. Kudo et al, IEEE, Photonics Technology Letters, July, 1998, pp. 929 to 931. The proposed technology is very effective as the method of fabricating the light sources for the WDM since it is possible to obtain EA modulator integrated DFB lasers having 40 (forty) wavelengths from a single semiconductor substrate. In the proposed technology, gratings having same pitch are arranged in one direction of the substrate as shown in FIG. 18.
However, the gratings have the same pitch with respect to one direction of the substrate as shown in FIG. 18, that is, elements having different oscillation wavelengths are arranged according to some linear function, that is, linearly with respect to a certain direction of the substrate as shown in FIG. 19(a). Therefore, when the elements are formed on a circular substrate, the number of elements formed in a center portion of the substrate becomes different from that of the elements formed in an edge portion thereof. Thus, the number of elements that can be fabricated on a single substrate varies substantially depending upon oscillation wavelength as shown in FIG. 19(b). Therefore, the proposed technique is inadequate as the fabrication method of light source for WDM.
An object of the present invention is to provide a fabrication method for fabricating optical semiconductor devices including DFB laser diodes having different oscillation wavelengths, EA modulators having different band gap wavelengths of absorption layers thereof or elements formed by the DFB laser diodes monolithically integrated with the EA modulators on a circular semiconductor substrate such as a 2-inch InP substrate, the number of semiconductor elements having a different one of operation wavelengths (oscillation wavelengths or absorption wavelengths), is constant. Particularly, an object of the present invention is to provide a fabrication method with which a constant number of elements for each operation wavelength can be obtained most efficiently from a usual circular substrate.
An according to the present invention, an optical semiconductor device fabrication method for simultaneously fabricating optical semiconductor devices having different operation wavelengths (operation energy) on a single circular semiconductor substrate is featured by that the number of elements having each of the operation wavelengths can be made constant.
A first aspect of the present invention resides in a method for fabricating an optical semiconductor device on a circular semiconductor substrate, which is featured by comprising parabolically changing an operation band gap energy (band gap wavelength) of the optical semiconductor device from a center portion of the circular semiconductor substrate toward an outer periphery of the circular semiconductor substrate.
A second aspect of the present invention resides in a method for fabricating a semiconductor laser on a circular semiconductor substrate, which is featured by comprising parabolically changing an oscillation wavelength of the semiconductor laser from a center portion of the circular semiconductor substrate toward an outer periphery of the circular semiconductor substrate.
A third aspect of the present invention resides in the method for fabricating a semiconductor laser according to the second aspect, which is further featured by that the semiconductor laser is a DFB-LD (distributed feedback laser diode) or a DBR-LD (distributed Bragg reflector laser diode).
A fourth aspect of the present invention resides in the method for fabricating a semiconductor laser according to the third aspect, which is further featured by that a pitch of gratings formed in a waveguide or in the vicinity of the waveguide is parabolically changed from a center portion of the circular semiconductor substrate toward an outer periphery of the circular semiconductor substrate.
A fifth aspect of the present invention resides in the method for fabricating a semiconductor laser according to the third or fourth aspect, which is further featured by that Ebxe2x88x92Eg is constant in a range from xe2x88x925 meV to +15 meV and, preferably, from xc2x10 meV to +10 meV, where Eb is energy of Bragg wavelength (laser oscillation wavelength) determined by the gratings pitch and Eg is energy of a gain peak wavelength of an active layer of the laser.
A sixth aspect of the present invention resides in a method for fabricating a semiconductor optical modulator of an EA type, on a circular semiconductor substrate, which is featured by comprising parabolically changing a band gap wavelength of an optical absorption layer of the semiconductor optical modulator from a center portion of the circular semiconductor substrate toward an outer periphery of the circular semiconductor substrate.
A seventh aspect of the present invention resides in a method for fabricating a semiconductor optical integrated device having a DFB-LD monolithically integrated with a semiconductor optical modulator of an EA type or a DBR-LD monolithically integrated with a semiconductor optical modulator of an EA type, on a circular semiconductor substrate, which is featured by comprising parabolically changing both a band gap wavelength of an optical absorption layer of the semiconductor optical modulator and an oscillation wavelength of the DFB-LD or the DBR-LD from a center portion of the circular semiconductor substrate toward an outer periphery of the circular semiconductor substrate.
An eighth aspect of the present invention resides in the method for fabricating a semiconductor laser according to the seventh aspect, which is further featured by that Emxe2x88x92Eb is constant in a range from 20 meV to 40 meV and, preferably, from 25 meV to 35 meV, where Em is band gap energy of the optical absorption layer of the EA modulator and Eb is energy of the DFB-LD or DBR-LD.
A ninth aspect of the present invention resides in a method for fabricating an optical semiconductor device on a circular semiconductor substrate, which is featured by comprising growing a semiconductor layer constituting the optical semiconductor device by MOVPE (metal-organic vapor phase epitaxy) and changing a temperature of the semiconductor substrate such that the band gap wavelength of the epitaxially grown layer is parabolically changed in a surface of the semiconductor substrate from a center portion of the semiconductor substrate toward an outer periphery thereof.
A tenth aspect to the present invention resides in a method for fabricating an optical semiconductor device on a circular semiconductor substrate, which is featured by comprising of growing a semiconductor layer constituting the optical semiconductor device by selective MOVPE using a dielectric film as a growth blocking mask and changing a width of the growth blocking mask such that the band gap wavelength of the epitaxially grown layer is parabolically changed in a surface of the semiconductor substrate from a center portion of the semiconductor substrate toward an outer periphery thereof.
An eleventh aspect of the present invention resides in a method for fabricating an optical semiconductor device on a circular semiconductor substrate, which is featured by comprising of growing a semiconductor layer constituting said optical semiconductor device by selective MOVPE using a dielectric film as a growth blocking mask and changing a width of an opening portion of the growth blocking mask such that the band gap wavelength of the epitaxially grown layer is parabolically changed in a surface of said semiconductor substrate from a center portion of the semiconductor substrate toward an outer periphery thereof.
A twelfth aspect of the present invention resides in the method of fabricating an optical semiconductor device according to any of the first to eleventh aspects, which is further featured by that the parabolic distribution of the operation and gap energy is approximated by a plurality of steps.
A first effect of the present invention is that, in the fabrication method of an optical semiconductor device for forming elements having different operation wavelengths on a circular semiconductor substrate, the number of elements having each of operation wavelengths obtainable from the semiconductor substrate can be efficiently made constant.
The reason for this is that the operation wavelengths of the optical semiconductor device are changed parabolically from a center portion of the substrate to an outer peripheral portion of the substrate.
A second effect of the present invention is that an optical semiconductor device applied to a DFB laser can operate highly uniformly with low threshold current and high efficiency at any of different oscillation wavelengths thereof.
The reason for this is that it is possible to always define an energy difference between DFB oscillation wavelength defined by the grating pitch and the gain peak wavelength defined by the crystal structure within a constant range.
A third effect of the present invention is that an optical semiconductor device applied to an EA modulator integrated DFB laser can operate highly uniformly with low threshold current, high efficiency and high light extinction ratio, at any of different oscillation wavelengths thereof.
The reason for this is that it is possible to always define an energy difference between DFB oscillation wavelength defined by the grating pitch and the gain peak wavelength defined by the crystal structure and an energy difference between the DFB oscillation wavelength and the band gap wavelength of the absorption layer of the modulator within a constant range.